This invention relates to an electronically controlled automatic transmission and a method of controlling the transmission and, more particularly, to an electronically controlled automatic transmission having functions capable of detecting any malfunction of a transmission output speed sensor or vehicle speed sensor used to control the transmission, as well as fail-safe functions associated therewith.
In an electronically controlled automatic transmission with a single vehicle speed sensor of the type already known, there are times when a computer may determine that the vehicle speed is zero even the vehicle is running, if the vehicle speed sensor malfunctions and fails to output a vehicle speed signal. In such a case, the computer determines that the vehicle is in a stationary state and sets the automatic transmission (hereinafter referred to simply as "transmission") to the first speed, which of course impairs the smooth running of the vehicle. One type of system designed to cope with this problem makes use of independent vehicle speed signals supplied via two lines, that is, an output speed signal from a vehicle speed sensor incorporated in the transmission and a speed signal from a vehicle speed sensor incorporated in a speedometer. This provides suitable redundancy and improves the reliability.
As described above, it is necessary for a transmission system to have redundancy by, for example, making use of two vehicle speed signals supplied via different lines in order to avoid any untoward influence on the smooth running of the vehicle due to malfunctioning of one of the speed sensors.